which radiate to the centre. If the space to be covered is not square the broader arches intersect at a higher level, while the narrow arches are not stilted, but kept down so as to receive the dome surface, and in this case the narrow arches are not semicircular5 but segmental. Where the difference in size between the two sides was not great., the difficulty presented was easily overcome by the Byzantine builder^ who in the later buildings, at any rate,,
DIAGRAM OF THE VAVLT1NG IN THE OUTER NARTHCX OT 5 SAVIOUR IN THE CHORA-
rarely built anything within four inches of its geometrical position. Where the difference was too great it was frankly accepted, and we find segmental arches at the narrow ends.
The vaulting of the outer narthex of S. Saviour in the Chora illustrates this fully (Fig. 11). Though some of the bays of that narthex are oblong and others almost square all are covered with dome vaults. The almost square bays, although their sides vary considerably? are covered precisely